Carbon fibers such as carbon nanofibers are promising materials for many possible applications, e.g. conductive and very strong composites, energy stores and converters, sensors, field emission displays and radiation sources and also nanosize semiconductor elements and testing points (Baughman, R. H. et al., Science 297:787-792 (2002)). Another promising application is catalysis using carbon nanofibers as catalysts or as supports for heterogeneous catalysts (de Jong, K. P. and Geus, J. W., Catal. Rev.-Sci. Eng. 42:481-510 (2000)) or as nanosize reactors for catalytic syntheses (Nhut, J. M. et al., Appl. Catal. A. 254:345-363 (2003)). It is frequently necessary to modify the surface either chemically or physically for the abovementioned applications. For example, complete dispersion of the nanofibers in a polymer matrix and the resulting strong interaction between fiber and matrix is advantageous in composites (Calvert, P., Nature 399:210-21 (1999)). When used as catalyst supports, foreign atoms have to be deposited on the nanofibers. Anchor points such as functional groups or defects are necessary for this purpose. To achieve this, the inert surface of the untreated (“as-grown”) nanofibers has to be modified (Xia, W. et al., Chem. Mater. 17:5737-5742 (2005)). For use in the sensor field, bonding of chemical groups or immobilization of a protein having specific recognition centers to/on the nanofibers is necessary. This is generally realized by production of functional surface groups or surface defects (Dai, H., Acc. Chem. Res. 35:1035-5742 (2002)).
Motivated by the promising possible applications, extensive studies on the surface modification and functionalization of carbon nanofibers have been carried out in the last 10 years. Among all these methods, the most intensive research has been carried out on covalent surface functionalization which is generally based on strong oxidants such as nitric acid, oxygen plasma, supercritical fluids, ozone and the like and, for example, subsequent side chain extension (Banerjee, S. et al., Adv. Mater. 17:17-29 (2005)). These oxidation methods usually increase the oxygen content of the surface, with visible physical modifications also being able to be achieved by appropriate selection of parameters. These physical changes are limited to two- or three-dimensional surface defects having unforeseeable structures in unknown positions. Under extreme conditions, for example a mixture of concentrated sulfuric acid and nitric acid, nanofibers are split into smaller fibrous units (Liu, J. et al., Science 280:1253-1256 (1998)). Identification of the surface defects remains a challenge because of the small dimensions and the curved surface of carbon nanofibers (Ishigami, M. et al., Phys. Rev. Lett. 93:196803/4 (2001)). Scanning tunneling microscopy (STM) is a very effective tool here (Osváth, Z. et al., Phys. Rev. B. 72:045429/1-045429/6 (2005)). Fan and coworkers have identified chemical surface defects by means of atomic force microscopy (AFM) using defect-sensitive oxidation with H2Se (Fan, Y. et al., Adv. Mater. 14:130-133 (2002)). In Xia, W. et al., Chem. Mater. 17:5737-5742 (2005), the alteration of the surface of carbon nanofibers is effected by deposition of cyclohexane on iron-laden carbon nanofibers. However, these secondary carbon nanofibers (tree-like structures composed of trunk and branches) are not functionalized and the surface modifications obtained cannot be used for loading with functional molecules.
The above problems apply analogously to carbon microfibers, e.g. carbon fibers produced from polyacrylonitrile (PAN) and composed of fiber bundles up to millimeter ranges, which are employed as continuous fibers in modern high-performance composites.
Despite the numerous efforts to modify the surface of carbon fibers such as carbon nanofibers, functional surface groups or surface defects have to the present time not been able to be introduced in a targeted manner by means of any of the abovementioned methods.